This invention relates to a new and improved fuel supply apparatus and more specifically to a booster pump assembly which is utilized to supply fuel to an aircraft engine.
During operation of a supersonic aircraft, heat rejection to the fuel is such that when there is a low fuel flow rate, the booster pump must deliver the fual at a higher pressure than when there is a high fuel flow rate. These requirements make the utilization of a fixed-geometry pump assembly or a speed-control approach to booster pump construction almost impossible. If a high flow rate booster pump impeller having a large passaget-to-diameter ratio, is driven at a substantially constant speed to supply both the high and low fuel flow rate requirements of an engine, the attendent recirculation power required at a relatively low fuel flow rate is the same or greater than the power consumed at the maximum fuel rate. Of course, this results in relatively inefficient booster pump operation.
Inefficient booster pump operation becomes particularly acute in the case of supersonic military aircraft. In supersonic military aircraft, the booster pump is required to supply relatively high fuel flow rates during take off and/or during afterburner operation. However, these conditions are present during only approximately 5 percent of flight time. Therefore, the booster pump has excess capacity during the remaining 95 percent of the flight time.
During 95 percent of supersonic military aircraft flight time, approximately 2 to 10 percent of booster pump capacity is required to satisfy the fuel requirements of the engine. Therefore, during the large majority of the engine operating time, a booster pump having a fixed-geometry impeller with a large passage-to-diameter ratio is inefficient in operation. In order to solve this problem, it has been proposed to use variable-geometry impellers in the manner disclosed in U.S. Pat. No. 2.950,686 and in U.S. Pat. No. 3,806,278. However, the complexity and/or inherent operating characteristics of these variable-geometry pumps have made them less than completely satisfactory.